Multi-path reception conditions give rise to ghosts in NTSC television reception. Ghost signals which arrive over a transmission path of lesser length than the strongest or "principal" signal are referred to as "pre-ghosts", and the ghost images they cause in a received television image appear to the left of the desired image. Pre-ghosts occurring in off-the-air reception can be displaced as much as 6 microseconds from the "principal" signal, but typically displacements are no more than 2 microseconds. In cable reception direct off-the-air pick-up can precede the cable-supplied signal by as much as 30 microseconds, however. Ghost signals which arrive over a transmission path of greater length than the strongest or "principal" signal are referred to as "post-ghosts", and the ghost images they cause in a received TV image appear to the right of the desired image. Typically, the range for post-ghosts extends to 40 microseconds displacement from the "principal" signal, with 70% or so of post-ghosts occurring in a subrange that extends to 10 microseconds displacement.
In cable reception the differential delays that ghosts exhibit with respect to the principal received signal are usually very short, leading to departure from flat amplitude response and uniform group delay in the transmission/reception channel. These ghosts are referred to as "micro-ghosts" to distinguish them from "macro-ghosts" exhibiting longer differential delays of at least a microsecond with respect to the principal received signal that are encountered in terrestrial broadcast DTV signals received over the air.
Equalization filtering can be carried out using a multiple-tap finite-impulse-response (FIR) digital filter. The weighting coefficients of such a filter can be adjusted to suppress the responses to multi-path signals which exhibit differential delay with respect to the principal received signal. Since macro-ghosts are generally spaced apart from each other and from the principal received signal, it is a common practice to use filters with sparsely weighted coefficients for suppressing them, cascading such filters FIR filters with more densely weighted coefficients used for suppressing micro-ghosts. The filters with sparsely weighted coefficients are sometimes referred to as ghost-reduction filters, and the filters with more densely weighted coefficients are sometimes referred to as equalization filters. In this specification the term "channel equalization filtering" is used generically to refer to both types of filter and to cascade connections of such filters. Often the ghost-reduction filters with sparsely weighted coefficients are themselves cascade connections of an infinite-impulse-response (IIR) recursive digital filter and an FIR digital filter, each comprised of programmable bulk delay elements between successive taps from which differentially delayed signals are extracted for programmable weighting in weighted summation digital filtering procedures. Procedures for adjusting the coefficients of cascaded ghost-reduction and micro-ghost equalization filters are described by C. B. Patel and J. Yang in U.S. Pat. No. 5,331,416 issued Jul. 19, 1994 and entitled "METHODS FOR OPERATING GHOST-CANCELATION CIRCUITRY FOR TV RECEIVER OR VIDEO RECORDER". Apparatus and procedures for accumulating training signals for NTSC analog television signals are described by C. B. Patel and J. Yang in U.S. Pat. No. 5,600,380 issued Feb. 4, 1997 and entitled "GHOST-CANCELATION REFERENCE SIGNAL ACQUISITION CIRCUITRY, AS FOR TV RECEIVER OR VIDEO RECORDER".
Similar multi-path reception conditions obtain in digital television (DTV) systems. The effects of ghosts are not directly observable on the television viewing screen but instead interfere with the data slicing procedures employed to recover data from baseband symbol coding regenerated at the DTV receiver responsive to received DTV signals. Ghost suppression is effected by digital filtering techniques similar to those used with NTSC signals.
Receivers for DTV signals that digitize these signals after conversion to a final intermediate-frequency band and before demodulating the converted signals to regenerate baseband symbol coding are described in the inventors' U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled "DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER". The receivers disclosed in U.S. Pat. No. 5,479,449 include equalization filtering, which is performed on the regenerated baseband symbol coding. U.S. Pat. No. 5,479,449 indicates a preference for adaptive weighting being done dependent on a ghost cancellation reference signal component of the regenerated baseband symbol coding. This method of adaptive weighting is preferred for initial adjustment of weighting in the channel equalization filtering as indicated in the inventors' and others' U.S. Pat. No. 5,648,987 issued Jul. 15, 1997 and entitled "RAPID-UPDATE ADAPTIVE CHANNEL-EQUALIZATION FILTERING FOR DIGITAL RADIO RECEIVERS, SUCH AS HDTV RECEIVERS". Thereafter, as indicated in U.S. Pat. No. 5,479,449 the adaptation of the channel-equalization filtering can be performed on a decision-directed basis using all received symbols, which better permits the tracking of changing multipath conditions.
The inventors point out that performing equalization on the digitized final intermediate-frequency (final-IF) signal, rather than on the regenerated baseband symbol coding, is preferable for a number of reasons. First, there is no spectrum folding of the differentially delayed signals prior to equalization, to complicate equalization of the double-sideband portions of the DTV signal. Second, the appropriate delay of low baseband frequencies is not difficult to implement. Third, the effects of the local oscillator AFPC loop help, rather than interfere with, equalization procedures. Fourth, equalization can be effected on co-channel NTSC as well as on the DTV signals without need for separate equalization filtering, as required if baseband equalization of the two types of signal is to be done; this is helpful when co-channel NTSC signals are to be analyzed for their effects on DTV signals.
Performing equalization on the digitized final intermediate frequency presents the problem that the channel equalization filter response being in the passband is unsuitable for the generation of adjustments to the adaptive weighting coefficients therein. The decision-feedback techniques used in prior art quadrature-amplitude-modulation (QAM) data communications receivers for demodulating the passband equalizer response, generating decision-feedback error signal in the baseband, computing weighting coefficients for a hypothetical baseband equalizer, and then applying lowpass-to-bandpass transformation procedures to generate weighting coefficients for the passband equalizer are inappropriate for VSB data communications receivers because the lowpass-to-bandpass transformation procedure is inapplicable.